Applicants claim the priority of Japanese patent application, Ser. No. 11-300118, filed Oct. 21, 1999.
This invention relates to an acceleration device, and more particularly to a carburetor acceleration device for a two-cycle engine.
Fuel from a carburetor for a two-cycle engine is fed via negative pressure into an air intake passage where the fuel mixes with the air and is then drawn into a crankcase. From the crankcase, the fuel-and-air mixture is drawn into a combustion chamber and burned. During engine acceleration the suction, or negative pressure, drawing the fuel and air mixture decreases. Therefore, less fuel is drawn into the air intake passage at a time when more fuel is actually required for smooth acceleration. Consequently, two cycle engines have been known to incorporate auxiliary acceleration pumps which use negative pressure to boost the delivery of fuel during acceleration periods.
Air pollutants from the exhaust of the two cycle engine are typically much greater than that of a four-cycle engine, because the two cycle engine does not completely bum the fuel within the combustion chamber. To alleviate some of the air pollutant concerns for two cycle engines, the industry is designing toward a leaner fuel to air mixture, and therefor a cleaner bum. Unfortunately, use of a leaner fuel to air mixture causes fuel starvation during engine acceleration periods. Sudden acceleration from idle of a cold engine may result in a stall due to lack of sufficient fuel. Moreover, use of the common auxiliary acceleration pump which is dependent upon negative pressure, is not responsive for a lean mixture engine because negative pressure is lacking during acceleration periods.
An acceleration device of a carburetor provides additional fuel to a two-cycle engine brought on by decreasing negative pressure during acceleration conditions. A carburetor body houses a scavenging passage and an air intake passage opened and closed via a scavenging valve and a throttle valve respectively. The scavenging and throttle valves are preferably integral to a single rotary dual valve and share a common axis of rotation. During steady engine operating conditions, fuel is supplied from a substantially constant pressure fuel supply chamber through a fuel supply tube and into a throttle hole of the throttle valve. The fuel is drawn from the throttle hole via negative pressure of the air intake passage when the intake passage is in communication with the throttle hole. During engine acceleration conditions, additional fuel is pushed into the throttle hole by inward movement of a diaphragm into the fuel supply chamber.
Preferably, a membrane disposed between a pump chamber or chamber and an actuation chamber or chamber of an acceleration pump pushes air into or increases the pressure in an air reference chamber housed within the carburetor body and communicating with the diaphragm of the fuel supply chamber. The membrane is actuated when a compressed resilient member, normally held back by a vacuum within the actuation chamber, pushes the membrane into the pump chamber when the vacuum is lost during engine acceleration conditions. The pushed air, in turn, forces the diaphragm into the fuel supply chamber. The vacuum within the actuation chamber is created by a suction from the scavenging passage during steady state engine operation.
Objects, features and advantages of this invention include providing a fuel acceleration device which is actuated by a sudden increase in pressure within a carburetor scavenging passage. The acceleration device thereby provides smooth acceleration of a lean burn two cycle engine even during cold operation, improved fuel efficiency and decreased engine emissions.